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AIR-SEA BOUNDARY PROCESSES 

 Walter H. Munk 



INTRODUCTION 



From a cause -and -effect point of view, nearly all subject matters that 

 have been discussed during this symposium can be traced eventually to a trans- 

 mission of energy, in one form or another, through the air- sea boundary. 

 Transmission through the sea bottom amounts to only 10"4 of that through the 

 surface. If the surface boundary were impervious to energy transmission, there 

 could hardly be any motion in the sea, and there could certainly be no marine 

 life. It is therefore worthwhile to know something about the transmission char- 

 acteristics of this boundary. 



SOLAR RADIATION 



Let us start with a simple problem. Solar radiation upon the sea surface 

 is partly transmitted, partly reflected. For normal incidence, about 2 per cent 

 is reflected, for glancing incidence, nearly all is reflected. The dependence 

 of reflectivity on the angle of incidence is given by Fresnel's law. Except for 

 some uncertainty associated with the polarization of the incoming radiation, the 

 reflection coefficient can be computed for any angle of incidence with an accura- 

 cy unusual in oceanographical problems. The situation would be altogether 

 satisfactory i£ the sea surface were flat. But this situation exists only approxi- 

 mately on a calm day, and it is far from true on a rough day. Suppose there is 

 a 30 mph wind, and the sun is 40° above the horizon. Then only half the in- 

 coming rays strike the sea surface at angles of incidence between 30° and 50° ; 

 for the other half the angle is either less than 30° or more than 50°. The over- 

 all reflection is now different from what it would be on a calm day. The effect 

 of the ruffling is, so to say, a smudging of Fresnel's curve. To compute the 

 reflection of solar radiation, we need to know not only the sun's elevation, but 

 also the wind. 



But this is not the only effect of the ruffling of the sea surface. The 

 transmitted light is not a steady uniform light, but varies, depending upon the 

 instantaneous configuration of the sea surface. Relatively low light intensity is 

 interrupted by the bright flashes of caustic lines. It is in this flashing and 

 flickering space, not under the steady radiation of a laboratory lamp, that the 

 photosynthesis of diatoms produces nearly all organic material on the earth. 



Since the incoming radiation is so greatly modified by the sea surface, 

 then in turn optical methods should provide a suitable means for studying the ge- 

 ometry of the sea surface. This idea is far from new. In 1820, Spooner, in a 

 letter to Baron de Zach, described his observations of the sun setting over the 

 Tyrrhenian Sea. He reasoned that if the sea were flat calm, there would be a 

 single mirror-like image of the sun on the water. But the fact that there were 



